1. Field of the Invention
The present invention relates to a stator for a starter motor used in vehicles and the like, and more particularly, to a stator for a starter motor having a cylindrical yoke.
2. Description of the Related Art
FIG. 7 is a perspective view showing a structure of a stator for a conventional starter motor; FIG. 8 is a plan view of the stator; and FIG. 9 is a perspective view showing an assembly process of the stator.
In a conventional stator 101 shown in FIGS. 7 and 8, four poles 103 are disposed inside a cylindrical yoke 102. Then, a field winding 105 is wound around each of the poles 103.
In the conventional stator constructed as described above, the yoke 102 is arranged as a cylindrical yoke (xe2x80x9cbent yoke portionxe2x80x9d) by rounding a sheet member and welding the joints thereof. Spigot portions 104, which are formed at the ends of the yoke 102 in the axial direction thereof so as to be fitted to brackets (not shown), are processed by machining.
The poles 103 are made in a different process by, for example, forging or the like and mounted on the yoke 102 later. Further, the field windings 105 are previously wound in a different process and the spaces between wires are previously subjected to insulation processing by powder coating or the like. The surface of the yoke 102 cannot be previously subjected to rustproof processing because it is bent, welded, drawn and machined. Thus, the yoke 102 is coated with paint upon completion of the processing thereof.
The conventional stator constructed as described above is assembled in such a manner that, first, the field coils 105, which have been previously wounded and subjected to the insulation processing, are mounted on the poles 103 which are mounted on the yoke 102 through mounting holes 110 defined thereto by caulking, screws or the like.
In the stator for the starter motor constructed as described above, the yoke 102 must have a predetermined thickness because a yoke made of a thin sheet member cannot achieve its function. Thus, the yoke 102 is made in such a manner that a thick sheet member is cut off by a large press and gradually rounded so as to be formed in a cylindrical shape, then the cylindrical thick sheet member is welded by a welder and subjected to a drawing process to provide it with accuracy by making the deformed cylindrical member to a perfect cylindrical member. Thereafter, the spigot portions 104 at the ends of the cylindrical member in the axial direction thereof are machined, the mounting holes 110 are drilled to the cylindrical member and painting is applied thereto.
The manufacture of the conventional so-called xe2x80x9cbent yoke portionxe2x80x9d made as described above requires many dedicated apparatuses such as a large press, welder, surface processing apparatus, outside diameter drawing machine, lathe, drilling machine, painting machine, and the like. Further, many processes are necessary to manufacture the xe2x80x9cbent yoke portionxe2x80x9d and the respective processing machines are large in size, from which a problem arises in that the manufacturing cost of the xe2x80x9cbent yokexe2x80x9d is expensive.
An object of the present invention, which was made to solve the above problem, is to provide a stator for a starter motor the manufacturing cost of which can be reduced.
In a stator for a starter motor according to the present invention which includes an approximately cylindrical yoke, poles disposed inside the yoke, and field windings wound around the poles, the yoke is formed of a plurality of cylindrical cores having a different diameter and stacked in a radial direction.
It is preferable that the plurality of cylindrical cores be manufactured by a clinch method, respectively.
It is preferable that joints used in the clinch method are located so that they are not overlapped in a circumferential direction.
It is preferable that the axial direction of the innermost core of the plurality of the cylindrical cores be longer than those of the other cores.
It is preferable that the axial direction of the outermost core of the plurality of cylindrical cores be longer than those of the other cores.
It is preferable that the axial direction of the intermediate core of the plurality of cylindrical cores be longer than those of the other cores.
It is preferable that only the outermost core of the plurality of cylindrical cores be composed of a plated steel sheet.
It is preferable that the plating thickness of the outermost core of the plurality of cylindrical cores be larger than those the other cores.
Further, it is preferable that only the outermost core of the plurality of cylindrical cores is composed of a coated steel sheet.